Frequency voltage regulator



Filed Oct. 30, 1947 INVENTOR. JUHN E 15/775195 UN Patented Nov. 28, 1950 UNITED STATES PATENT OFFICE FREQUENCY VOLTAGE REGULATOR John F. Emerson, Hasbrouck Heights, N. J., assignor to Bendix Aviation Corporation, Teterboro, N. 1., a corporation of Delaware Application October 30, 1947, Serial No. 783,008

8 Claims. 1

This invention relates to a frequency regulartor, and more particularly to a carbon pile regulator in which the carbon pile resistance is in the field circuit of a motor driving the generator developing the voltage, the frequency of which is to be regulated.

An object of my present invention is to provide a regulator in which the resistance of a carbon pile in the field circuit of the motor is varied in accordance with the frequency of the alternating current developed by the generator driven by the motor.

A further object of the present invention is to provide a frequency regulator of the character indicated in which the average rectified value of the voltage developed by the generator is used as a reference and to which a voltage responsive to the frequency of the developed voltage is added to vary the resistance of a carbon pile in the circuit of the motor driving the generator to maintain the speed of the motor at a constant value.

Another object of my invention is to provide a frequency regulator of the general character indicated in which signal voltages responsive to the frequency and to the amplitude of the generated voltage, are applied to a vacuum tube circuit to control the output thereof for controlling the speed of a motor driving the generator developing the voltage.

Still another object of my invention is to provide a regulator of the character indicated in which the resistance of a carbon pile is varied in accordance with the output of a vacuum tube circuit controlled by the frequency and substantially independent of the voltage, in which the changes in the carbon pile resistance tend to stabilize the value thereof.

Still a further object of this invention is to provide a regulator of the character indicated in which the flow of excessive armature current is prevented by limiting the maximum value of the carbon pile resistance through a circuit attenuating excessive signal voltages.

Yet a further object of the present invention is to provide a regulator of the character indicated in which the flow of excessive armature current is further prevented by the change in carbon pile resistance aflecting the bias voltage of the vacuum tube circuit.

Yet another object of this invention is to provide a regulator in which the amplitude and frequency of the generated voltage is unaffected by the changes in the motor voltage supply.

Still a further object is to provide a regulator of the character indicated in which fall safe" provisions will reduce the speed of the motor upon failure 11 the regulating system.

Still another object of the invention is to provide a highly improved regulator of the character indicated which shall be simple in construction, relatively inexpensive to manufacture, positive in its action and automatic in its operation, which shall be unaffected by temperature changes in the circuits thereof and which is practical and efficient to a high degree in use.

Other objects of this invention will be obvious and in part hereinafter pointed out.

I accomplish the several objects of the invention by providing a carbon pile resistor in the field circuit of a shunt wound D. C. motor driving an A. C. generator. The resistance of the carbon pile is varied to control the speed of the motor by an electronic control circuit connected to the output of the generator. The control circuit is essentially in two sections, one section providing a signal voltage proportional to the generator voltage while the other section provides a signal voltage also proportional to the generator voltage but modified in response to the frequency of the generator voltage. The two signal voltages thus derived are applied to the input of the electronic circuit in series opposed relation. The components of the generated voltage tend to balance out, leaving the control of the electronic circuit sensitive to only the frequency variations in the generator voltage. The resistance of the carbon pile is thus varied in accordance with the frequency of the generator voltage to maintain the operation of the motor at a constant, predetermined speed.

Referring now in detail to the single figure of the accompanying drawing in which there is shown a schematic wiring diagramv of an embodiment of my invention, the numeral ll designates a shunt wound D. C. motor having its armature connected across the leads I! and ll of a D. C. power supply It, the lead it being grounded as at II. The motor ll drives an A. C. generator It through a suitable shafting ll, said generator being connected at one side to the grounded lead I! and at the other side to a power lead ll.

A carbon pile regulator 2| is provided to control the speed of the D. C. motor III in accordance with the A. C. output of the generator It. The regulator 2| includes a resistance pile 2|, a pivoted armature 22, and a coil spring 23; the bias of the spring being such as to increase the pressure of the armature 22 on the pile 2| to trol coil 24. r .with the carbon pile resistance 2|, the magneto- 3 decrease the resistance thereof. Two magnetizing coils 24 and are also provided in aiding relation, the energization of which will attract the armature 22 against the bias of the spring 23 to reduce the pressure on the resistance pile 2|. The coil 24 will hereinafter be referred to as the control coil, while coil 25 will be referred to as the stabilizing coil.

One end of the stabilizing coil 25 is connected by a lead 28 to the motor shunt field 23 connected to the D. C. supply line l2. The other end of the stabilizing coil is connected by two leads 30 and 3| to the armatur 22 of the regulator. The armature 22 is electrically connected through the carbon pile 2| to a lead 32 connected at its other end to the grounded supply line I3.

The stabilizing coil 25 is poled to aid the con- The stabilizing coil being in series motive force develeped by the coil is dependent on the ohmic value of the resistance. The magnetomotive force of the coil decreases as the value of the carbon pile resistance increases. Thus a magnetizing current flowing through the control coil 24 will attract the armature 22 of the regulator. The resistance of the pile 2| will increase due to the decrease in pressure. The increase in resistance of the pile will decrease the magnetizing effect of stabilizing coil 25, tending to increase the pressure of the armature 22 on the pile slightly. Similarly, if the magnetizing current in control coil 24 should decrease, the pressure on the pile 2| will increase tending to decrease the resistance of the carbon pile. The magnetomotive force of the stabilizing coil will increase tending to decrease the pressure on the pile slightly. Thus. the magnetomotive force developed by the control coil 24 is offset slightly by that developed by the stabilizing coil 25 to stabilize the resistance value of the carbon pile 2 I.

It will be apparent that fluctuations in the supply voltage l4 will be partially compensated by the stabilizing coil 25. A reduction of the supply voltage will reduce the magnetomotive force of the coil tending to decrease the resistance of the carbon pile 2|. A greater current will then flow through the field winding 23. An increase in the supply voltage will tend to increase the magnetizing current through the coil 25 to increase the resistance of the carbon pile 2|.

Thus the stabilizing coil 25 will stabilize the resistance of the carbon pile 2| with respect to the magnetizing current in the control coil 24, and also aid, as .will hereinafter be described, in compensating for changes in motor speed due to fluctuations in the D. C. supply l4.

Means are now provided to energize the control coil 24 of the carbon pile regulator 20 for maintaining the speed of the motor In at a constant value.

To this end there is provided in the circuit illustrated a triode vacuum tube 35, having a cathode 36, a control grid 31, and a plat 38. The control coil 24 is connected to the plate 35 by a lead 33, and by a lead through two resistors 4| and 42 connected in parallel to the cathode 43 of a full-wave rectifier tube 44. The

rectifier tube is connected in a conventional manner across a centertapped secondary winding 45 of a transformer 46, the centertap being connected by a lead 43 to the cathode 36 of the control tube through a resistor to supply the bias voltage thereof. Theprimary winding 41 of the transformer is grounded at one end and connected to the A. C. power lead l8 at the other 4 end. The transformer 46 is provided with additional secondary windings 49 to supply the necessary filament voltages, as hereinafter described.

It is evident, that the conduction of the control tube 35 will cause an energizing current to flow through the control coil 24 to increase the resistance of the carbon pile 2| for varying the current through the field of the D. C. motor ID. The resistors 4| and 42 in series with the control coil 24 have a negative temperature coeflicient of resistance. Since the heating of the control coil by the magnetizing current therethrough and variation in amibent temperature affects the resistance thereof, the two resistors being in series and in thermal contact therewith will compensate for the changes in resistance to maintain the resistance of the control coil circuit constant.

The operation of the triode tube 35 is controlled by the A. C. output of the generator I5 through two parallel circuits connected to the power lead l8. One circuit will develop a D. C. voltage proportional to the average voltage developed by the generator and independent of the frequency, while the second circuit will develop a D. C. voltage proportional to the generator voltage but also affected by the frequency thereof. The two signal'voltages thus developed are combined and applied to the grid 31 of the control tube 35.

A transformer 50 having a grounded primary winding 5| is connected to the A. C. power line l8. The center-tapped secondary winding 52 of the transformer is connected in the conventional manner to the plates of a full-wave rectifier tube 53. The center tap of the secondary winding 52 is connected by a. lead 54 to the grid 31 of the control tube, said lead serving as a connection to one end of a resistor 55. The other end of the resistor 55 is connected by way of a lead 56 to the cathode of the rectifier tube 53. The A. C. voltage developed by the motor driven generator 5 is thus applied to the rectifier circuit described and is impressed as D. C. across the re sistor 55.

The D. C. component of the rectified generator voltage appearing across the resistor 55 is proportional to the average value of the A. C. voltage developed by the generator I6.

The second of the two control circuit for the tube 35 is by way of a frequency sensitive circuit connected to the A. C. power lead l8, generally designated by the numeral 60. This circuit comprises a series resonant circuit of a condenser 6|, an inductance 62, and a second condenser 63, one plate of the condenser 63 being grounded as at 69. The resonant circuit described is resonant below the desired frequency of the A. C. voltage generated. To increase the control sensitivity of the resonant circuit near the regulated frequency, a condenser 64 is connected across the inductance 62. The inductance and the shunt condenser form a high impedance at anti-resonance which is arranged to occur at a slightly higher frequency than resonance for the series circuit. The overall effect of the addition of the condenser 64 is to cause a rapid change in the response characteristic curve of the circuit 60 in a relatively narrow frequency band extending on either side of the regulated frequency.

Although the maximum' transmission (resonance) and minimum transmission (anti-resonance) will be affected by change in the losses of inductor 62, a frequency approximately midway between the frequencies for these two conditions will be substantiallyindependent of such 8 losses. This should be used for the regulated frequency to minimize temperature efleot.

A condenser 55 is connected in series with the condenser II to form a capacity voltage divider. The response of the frequency sensitive circuit ll described is the same as though the generator I. developed a lower voltage and the two condensers were connected in parallel.

The output of the frequency sensitive circuit II is tapped on as at point 55 on inductance I2 and appliedthrough a lead 61 to one side of the primary windings ll of a transformer Ill. The other end of the primary winding is connected by a lead H to the condenser 53 and to the ground connection 69. The center-tapped secondary winding 12 of the transformer 10 is connected in the usual manner to a full-wave rectifier tube II. The output voltage of the rectifier is applied by way of a lead 14 itnerconnecting the center tap of the secondary winding 12 with one end of a resistor said resistor being in series with the resistor 55. The lead 14 is grounded as at II. The lead 55 connected to the cathode of the rectifier completes the rectifier circuit to the other end of the resistor 15.

The D. C. component of the rectified generator voltage appearing across the resistor 15 is proportional to the voltage generated as modified by the frequency circuit 80. As the frequency of the generated voltage increases from zero, the voltage output of the rectifier 13 also increases until a maximum voltage is obtained. Maximum voltage will occur when the frequency is slightly less than the regulated frequency of the circuit 50. The rectifier output voltage will thereafter decrease sharply with increase in frequency until the regulated frequency is obtained, at which point the voltage output of rectifiers 53 and II will be very nearly equal. With increase in frequency, the output of rectifier 13 will continue to decrease.

The resistors 55 and 15 being connected in series opposition, the algebraic sum of the rectifier voltages impressed thereon will provide the grid voltage for operating the control tube 35. The polarities of these voltages are in opposition so that the voltage across 55 acts as a reference from which the voltage across 15 subtracts a greater or lesser amount depending upon the frequency of the generator. Thus the grid voltage applied will be dependent upon the frequency of the generated voltage and substantially independent of the amplitude thereof.

In starting the motor Ill it is desirable to obtain a high torque with a minimum of armature current. Since the spring It creates a pressure on the carbon pile 2| through the armature 22 of the regulator, the resistance of the carbon pile is" at a minimum. As the motor speeds up to a value consistent with the minimum resistance in the field circuit, the voltage developed by the generator It will operate the tube 35 to increase the resistance of the carbon pile. Since the peak of the output characteristic of the frequency sensitive circuit occurs at a frequency less than the regulated frequency, the output of the rectifier 13 applied across the resistor II will increase until the peak frequency is reached. Thus the grid voltage applied to the control tube II will increase in the positive direction with the voltage across the resistor 15. The plate current through the control coil 24 will likewise increase with the rise of motor speed. The energization oi the coil 24 will decrease the pressure on the carbon pile 2| increasing the pile resistance in vary the pressure on the carbon pile 2i.

the motor field circuit causing the motor to continue to accelerate. when the speed of the motor approaches that of the desired speed, the carbon pile resistance will be rapidly decreased due to the decrease in the plate current of the control tube in the control coil 2|. The decrease in carbon pile resistance will continue until the regulated frequency is attained.

Any deviation from the regulated frequency due to motor operation will increase or decrease the grid voltage applied to the control tube 35 to The change in the resistance in the circuit of motor field I! will vary the speed of the motor In to bring the frequency of the generated voltage to the regulated frequency.

For purposes hereinafter appearing it is desirable to obtain substantially D. C. voltage from the rectifier circuits. To this end a filter is connected in the grid lead 54 which comprises a choke ll with a condenser T1 in parallel therewith and a by-pass condenser 18. The choke and condenser 11 are anti-resonant at the principal ripple frequency in the output of the rectifiers. The by-pass condenser I8 connected by leads l8 and to the leads 5 and 14 respectively, has the lowest capacity consistent with the permissible ripple voltage at the grid 31 of the control tube. Unncessary filtering is to be avoided since it will increase the. recovery time and aggravate any tendency of the system to hunt.

To reduce the tendency of the circuit to hunt, an anti-hunt circuit, generally designated by the numeral II is connected into the grid circuit of the control tube.

Connected in the grid lead 54 is a resistor t2 having a second resistor 83 connected in parallel therewith through two condensers 84 and 85. A third resistor 86 is connected in series with the resistor 83, the other end thereof being connected to the grounded lead 14.

The resistor 82 will conduct the steady state current to the grid 31 of the control tube. If the motor ill accelerates or hunts about the regulated condition, the changes in the signal voltage due to the changing condition of the generated voltage will charge the condenser 84 and discharge through the resistors 83 and it. Thus an increment of the grid voltage determined by the values of the resistors 83, and condensers 84 and 85 will be added to the grid lead 54 through the condenser 85. The voltage applied will depend upon the rate of change of the frequency of the generated voltage. The anti-hunt component of the signal voltage will lead the signal voltage, so that a leading component of the signal voltage is applied to the grid lead 54 to correct for the inherent lagging response of the rest of the system. Hunting, therefore, by the motor it will be eliminated by the slight decrease or increase of the plate current flowing through the control coil 24, to decrease or increase the value of the carbon pile resistance in the motor field circuit in anticipation of the final signal voltage.

In order to prevent the flow of excessive armature current the maximum value of the carbon pile resistor 2| is limited by a limiting circuit provided in the grid circuit of the control tube 35 to limit the plate current thereof.

To this end a resistor ll is connected in the lead 54 in series with the resistor 82. A diode is also provided having a plate 5i and an indirectly heated cathode 92. The plate of the diode is connected by a lead 53 to the lead 54 while the cathode is connected by a lead 94 through the 7 series connected resistors 00. II, and 01 to the sround connection II.

Should the signal voltage applied to the grid 01 increase to a point where plate becomes positive with respect to the cathode 02, the diode 00 will conduct. The excess voltage will drop across the resistor 01 thereby limiting the grid voltage applied to the control tube 30. The plate current of the control tube is thus limited, limiting the maximum value of the carbon pile resistor 2|.

A further limiting circuit is provided to limit the maximum value of the carbon pile resistance 2| by developing a bias voltage in the control tube circuit to limit the plate current thereof.

To this end a second diode I00 is provided having a plate IM and an indirectly heated cathode I02. The plate IN is connected by a lead I03 to the junction point of the leads 30, 2| at the regulator armature 22. The cathode I02 is connected by a lead I04 through a resistor I05, and through the resistors 00. 07 to the ground point It. The cathode I02 is further connected by lead I06 to the lead 48, and through a resistor I0l to the cathode 06 of the control tube 35. A resistor I08 is interconnected between the plate 38 and the resistance'to minimize the increase in motor cathode 06 of thecontrol tube to carry a portion of the plate current thus permitting the use of a smaller tube for the control tube.

Examination of the circuit for the diode I00 will reveal a bridge circuit in which the resistance pile 2| forms one arm thereof, and the stabilizing coil and motor field winding 29 form a second and adjacent arm ofthe bridge. The resistor 95 forms a third arm opposite the resistance pile, while resistors 98 and 91 formthe fourth arm opposite the coil 25 and winding 29.

Should the resistance of the carbon pile 2| increase so that' 21 avi-R01 is greater than Ravin wherein R is the resistance of the element identilled by the subscript, the plate IOI of the diode will become positive with respect to its cathode. The diode I00 will then conduct to develop a bias voltage across the resistor I05. The bias voltage for the control tube being obtained from the plate current flowing through the resistor I01 and the voltage drop across the resistor 96, the development of a third bias voltage by the diode I00 across the series resistor I95 will increase the bias of the control tube 35. The plate current is thus limited, limiting the maximum value of the carbon pile resistor 2|.

A further feature of the cathode circuit of the control-tube described, is that compensations are made therein upon fluctuations in the D. C. supply I4.

Since the bias voltage drop across the resistor 06 is proportional to the D. C. supply voltage, a reduction in supply voltage will reduce the bias voltage of the control tube. The plate current will thus increase, to decrease the pressure of the armature 22. The carbon pile resistance will thus be increased to maintain the speed of motor I0. The decrease in field current under these conditions is correct in view of the decrease in armature current. An increase in the supply voltage on the other hand, will increase the bias of the control tube to decrease the carbon File speed.

Certain "fail safeprovisions are incorporated in the circuits hereinbefore described.

If a loss in current through the control coil 24 exists, the spring 23 will increase the pressure of the armature 22 on the carbon pile 2|. The spring thusminimizes the resistance of the carbon pile to reduce the motor speed.

Under normal operating conditions, the correct regulated speed of motor I0 will provide a voltage drop across the resistor 15 about 10% greater than the drop across the resistor 55. If no'signal voltage appears across the output circuits of the rectifiers 53 and 13, the conduction of the control tube 35 is reduced, thereby reducing the magnetizing current through the control coil 24.

Further, with the series connection of the rectifier filaments to the secondary winding 40 of the transformer 46, failure of either tube will cause both to stop conducting, permitting the control coil 24 to become deenergized to decrease the resistance of the carbon pile 2|.

It will thus be seen that there is provided a regulator in which the several objects of this invention are achieved and which is well adapted to meet the conditions of practical use.

As various possible embodiments might be made of the above invention, and as various changes might be made in the embodiment above set forth, it is to be understood that all matter herein set forth, or shown in the accompanying drawings, is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim as new and desire to secure by Letters Patent:

1. A control system comprising a direct current motor, an alternating current generator driven by said motor, a carbon pile resistor in the field circuit of said motor, a resiliently biased armature for said resistor for maintaining the ohmic value of said pile at a minimum, electromagnetic means for increasing the ohmic valu of the resistor upon energization thereof, a vacuum tube for controlling the energization of said electromagnetic means, a frequency sensitive circuit for said generator including resonant and anti-resonant circuits to provide a signal voltage responsive to the frequency and amplitude of the generated voltage, a circuit to provide a. signal voltage proportional to th generated voltage and circuit means including a filter circuit for applying the rectified signal voltages to the input of said tube to control the energization of said electromagnetic means.

2. A control system comprising a direct current motor, an alternating current generator driven by said motor, a carbon pile resistor inthe field circuit of said motor, a resiliently biased annature for said resistor for maintaining the ohmic value of said pile at a minimum, electromagnetic means for increasing the ohmic value of the resistor upon energization thereof, a vacuum tube for controlling the energization of said electromagnetic means, a frequency sensitive circuit for said generator including resonant and anti-resonant circuits, the output of which is peaked at a frequency other than that of the desired frequency to provide a signal voltage responsive to the frequency of the generated voltage, a circuit to provide a signal voltage proportional to the voltage generated, and circuit means for applying the rectified signal voltages across the input circuit of said tube, said circuit including an anti- I :9 hunt circuit to add an increment of the combined signal voltages to the input of said tube at a leading or. lagging phase angle dependent upon the rate of change of the frequencyof the generated voltag for stabilizing the speed of said motor.

3. A control system comprising a direct current motor, an alternating current generator driven by'said motor, a carbon pile resistor in the field circuit of said motor, a resiliently biased armature for said resistor for maintaining the ohmic value of said pile at a minimum, electromagnetic means for increasing the ohmic value of the resistor upon energization thereof, a vacuum tube for controlling the energization of said electromagnetic means, a frequency sensitive circuit for said generator including resonant and anti-resonant circuits, the output of which is peaked at a frequency other than that of the desired frequency to provide a signal voltage responsive to the frequency and amplitude of the generated voltage, a circuit to provide a signal voltage proportional to the voltage generated, and circuit means for applying the rectified signal voltages across the input circuit of said tube said circuit including an anti-hunt circuit to add an increto the frequency substantially independent of ment of the combined signal voltages to the in- I put of said tube at a leading or lagging phase angle dependent upon the rate of change of the frequency of the generated voltage for stabilizing the speed of said motor, said circuit including a diode tube for limiting the conduction of said first tube to prevent the flow of excessive armature current in said motor,

4. A control system comprising a direct cur-- rent motor, an alternating current generator driven by said motor, a carbon pile resistor in the field circuit of said motor, a resiliently biased armature for said resistor for maintaining the ohmic value of said pile at a. minimum, electromagnetic means for increasing the ohmic value of the resistor upon energization of said electromagnetic means, a frequency sensitive circuit for said generator including resonant and anti-resonant circuits, the output of which is peaked at a frequency other than that of the desired frequency to provide a signal voltage responsive to the frequency and amplitude of the generated voltage, a circuit to provide a signal voltage proportional to the voltage generated, and circuit means for applying the rectified signal voltages across the input circuit of said tube, said circuit including an anti-hunt circuit to add an increment of the combined signal votages to the input of said tube at a leading or lagging phase angle dependent upon the rate of change of the frequency of the generated voltage for stabilizing the speed of said motor, said circuit including a diode tube for limiting the conduction of said first tube to prevent the fiow of excessive armature current in said motor, a second diode tube, and abridge circuit including said carbon pile resistor in one arm thereof, theunbalance of said bridge operates said second diode to increase the bias of said first tube to further limit the flow of motor armature current.

5. A control system comprising a direct current motor, an alternating current generator driven by said motor, a carbon pile resistor in the field circuit of said motor to control the speed thereof, a resiliently biased armature for said resistor for maintaining maximum pressure on said pile resistor, ectromagnetic means adapted to decrease the pressure of said armature upon energization thereof, means for energizing said electromagnetic means by signal voltages responsive the amplitude of the generated voltage to control the motor speed at a predetermined value, and a second electromagnetic means for decreasing the pressure of said armature on said carbon pile, in series with said carbon pile resistor in the field circuit of said motor, to stabilize the speed of said motor at the predetermined speed.

6. A control system comprising a direct current motor, an alternating current generator driven by said motor, a carbon pile resistor in the field circuit of said motor, a resiliently biased armature for said resistor for maintaining the ohmic value of said pile at a minimum, electromagnetic means for increasing the ohmic value of the resistor upon energization thereof, a vacuum tube for controlling the energization of said electromagnetic means, means for controlling the conduction of said tube in accordance with the frequency substantially independent of the amplitude of the voltage developed by said generator to control the speed of said motor, and a second electromagnetic means for decreasing the pressure of said armature on said carbon pile,

. i series with said carbon pile resistor in the field circuit of said motor, to stabilize the speed of said motor at the predetermined speed.

7. A control system comprising a direct current motor, an alternating current generator driven by said motor, a carbon pile resistor in the field circuit of said motor, a resiliently biased armature for said resistor for maintaining the ohmic value of said pile at a minimum, electromagnetic means for increasing the ohmic value of the resistor upon energization thereof, a vacuum tube for controlling the energization of said electromagnetic means, a frequency sensitive circuit for said generator including resonant and anti-resonant circuits, the output of which is peaked at a frequency other than that of the desired frequency to provide a signal voltage responsive to the frequency and amplitude of the generated voltage, a circuit to provide a signal voltage proportional to the voltage generated, circuit means! for applying the rectified signal voltages across the input circuit of said tube, said circuit including an anti-hunt circuit to add an increment ofthe combined signal voltages to the input of said tube at a leading or lagging phase angle dependent upon the rate of change of the frequency of the generated voltage for stabilizing the speed of said motor, said circuit including a diode tube for limiting the conduction of said first tube to prevent the fiow of excessive armature current in said motor, and a second electromagnetic means for decreasing the pressure of said armature on said carbon pile, in series with said carbon pile resistor in the field circuit of said motor, to' stabilize the speed of said motor at the predetermined speed.

8. A control system comprising a direct current motor, an alternating current generator. driven by said motor, a carbon pile resistor in the field circuit of said motor, a resiliently biased armature for said resistor for maintaining the. ohmic value of said pile at a minimum, electromagnetic means for increasing the ohmic value of the resistor upon energization of said electromagnetic means, a frequency sensitive circuit for said generator including resonant and antiresonant circuits, the output of which is peaked at a frequency other than that of the desired frequency to provide a signal voltage responsive to the frequency and amplitude of the generated voltage, a circuit to provide a signal voltage proportional to the voltage generated, circuit means for applying the rectified signal voltages across the input circuit of said tube. said circuit including an anti-hunt circuit to add an increment of the combined signal voltages to the input or said tube at a leading or l ssins phase angle dependent upon the rate or change oi the frequency oi the generated voltage for stabilizing the speed of said motor, said circuit including a diode tube for limiting the conduction of said first tube to prevent the fiow or excessive armature current in said motor, a second diode tube. and a bridge circuit including said carbon pile resistor in one arm thereof. the unbalance of which operates said second diode to increase the bias of said first tube to further limit the flow 01 motor armature current, and a second electromagnetic means for decreasing the pressure or predetermined speed.

JOHN, I". EMERSON.

REFERENCES CITED The following references are of record in the file 01 this patent:

UNITED STATES PATENTS Number Name Date 1,333,662 Kazenmaier Mar. 18, 1920 1,899,193 Godsey Feb. 28. 1933 1,981,040 Gulliksen Nov. 20, 1994 2,001,557 Von Ohlsen May 14. 1935 2,284,049 Grabau June 2, 1942 2,420,312 Grabau May 13. 194'! 

